The contents of a Puppet manifest named params.pp
would ordinarily be the definition of a Puppet class whose simple name is "params" (or possibly of a defined type having that name). It is fairly common for modules to use such a class in an implementation of the "params class" pattern, wherein the purpose of the class is to provide, via its class variables, default values for the class parameters of other classes in the module.
I suppose, therefore, that what you propose to do is collect the names and values of the class's variables into a hash to test them, but such a test is outside the scope of rspec-puppet:
Rspec-puppet tests are there to test the behaviour of Puppet when it compiles your manifests into a catalogue of Puppet resources.
(Rspec-puppet Tutorial)
In other words, rspec-puppet is for testing the catalog compilation process and its results. If you examine the available matchers, you will see that this is their focus. The values of class variables that are not parameters (such as those of a params class) are not included as themselves in compiled catalogs, however, so they are not subject to testing with puppet-rspec. You can test whether those values are reflected in the parameters of resources that are included in the catalog, but the catalog contains no information whatever on variables that were available but not used.